Whole Grains

Summary

Whole-grain foods contain the entire grain, including the bran, the endosperm, and the germ. (More information)

Epidemiological studies have found that diets rich in whole grains are associated with reduced risks of cardiovascular disease and type 2 diabetes compared to diets high in refined grains. (More information)

Although the protective effects of whole grains against cancer are not as well-established as those against cardiovascular disease and type 2 diabetes, some epidemiological studies have found whole-grain intake to be associated with decreased cancer risk. (More information)

Diets rich in whole grains and fiber help prevent constipation and are also associated with decreased risk of diverticulosis. (More information)

The 2005 Dietary Guidelines for Americans recommend consuming a minimum of three servings of whole-grain products daily. (More information)

Introduction

Grains are seeds of plants belonging to the grass family. Species that produce edible grains include wheat, rice, maize (corn), barley, oats, and rye (1). An intact grain has an outer layer of bran, a carbohydrate-rich middle layer called the endosperm, and an inner germ layer ( Figure 1). Although not always intact, whole-grain foods contain the entire grain, including the bran, the endosperm, and the germ. Whole grains are rich in potentially beneficial compounds, including vitamins, minerals, and phytochemicals, such as lignans, phytosterols, and fiber. Most of these compounds are located in the bran or the germ of the grain, both of which are lost during the refining process, leaving only the starchy endosperm (2). Compared to diets high in refined grains, diets rich in whole grains are associated with reduced risks of several chronic diseases. The health benefits of whole grains are not entirely explained by the individual contributions of the nutrients and phytochemicals they contain. Whole grains represent a unique package of energy, micronutrients, and phytochemicals that work synergistically to promote health and prevent disease.

Disease Prevention

Type 2 diabetes mellitus

At least seven large prospective studies have found that higher whole-grain intakes are associated with significant reductions in the risk of developing type 2 diabetes mellitus (DM) over time (3-8). In the studies conducted in the United States, those who consumed an average of about three daily servings of whole-grain foods had a risk of type 2 DM that was 21-30% lower than those who rarely or never consumed whole grains (3-5). (See below for Examples of one serving of whole grains.) In Finland, the quarter of the population with the highest whole-grain intake had a risk of type 2 DM that was 35% lower than the quarter with the lowest intakes (6). A recent systematic review of six prospective cohort studies, including more than 286,000 participants, found that increasing daily intake of whole grains by two servings resulted in a 21% reduction in risk for type 2 DM (7). Insulin resistance is a condition of decreased insulin sensitivity that increases the risk of developing type 2 DM. In observational studies, higher whole-grain intakes have been associated with decreased insulin resistance (9) and increased insulin sensitivity (10) in people who do not have type 2 DM. In a controlled clinical trial that compared the effects of a diet rich in whole grains with a diet high in refined grains in overweight and obese adults, several clinical measures of insulin resistance were significantly lower after six weeks on the whole-grain diet compared to the refined-grain diet (11). Well-designed, long-term randomized controlled trials are necessary to determine whether whole grains are protective against the development of type 2 DM (12).

The refining process makes the carbohydrate in the endosperm of the grain easier to digest. Immediately after a meal, carbohydrate from refined grains elicits a higher and more rapid elevation in blood glucose, as well as greater demand for insulin (13). Over time, elevated blood glucose levels and compensatory increases in insulin secretion may lead to the development of type 2 diabetes. The glycemic index is a way of ranking the glucose-raising potential of carbohydrate in different foods. Foods made from whole grains generally have lower glycemic index values than foods made from refined grains (14). Substituting whole-grain foods for refined grain foods decreases dietary glycemic load which has been associated with decreased risk of type 2 DM (15, 16) and improved control of blood glucose levels in people who have diabetes (17). Thus, substituting low-glycemic index, whole-grain foods for high-glycemic index, refined-grain foods may substantially decrease the risk of developing type 2 diabetes. For more information, see Glycemic Index and Glycemic Load.

Cardiovascular disease

At least seven large prospective cohort studies have found that higher intakes of whole grains are associated with significant reductions in coronary heart disease (CHD) risk when compared to lower intakes (18-24). In general, those with the highest intakes of whole grains (about three servings daily) had a risk of CHD that was 20-30% lower than those with the lowest intakes even after adjusting the risk estimates for other heart disease risk factors. Whole-grain foods consumed in these studies included dark bread, whole-grain breakfast cereals, popcorn, cooked oatmeal, brown rice, bran, barley, and other grains like bulgar and kasha. A study that followed more than 85,000 male physicians for five years found that those who consumed at least one serving of whole-grain breakfast cereal daily had a risk of death from cardiovascular disease that was 20% lower than those who rarely or never consumed whole-grain cereal (23). A recent study in a multi-ethnic cohort found that whole-grain intake was inversely associated with intimal medial thickness of the common carotid artery, a marker for atherosclerosis(25). Higher intakes of whole grains have also been associated with a decreased risk of ischemic stroke (a stroke caused by the obstruction of a blood vessel that supplies the brain). A study that followed more than 75,000 women participating in the Nurses’ Health Study for 12 years found that women who consumed an average of almost three servings of whole grains daily had a risk of ischemic stroke that was more than 30% lower than women who rarely consumed whole grains (26). Moreover, a recent meta-analysis of seven prospective cohort studies found that those with higher whole-grain intakes (average of 2.5 daily servings) had a 21% lower risk of cardiovascular disease events when compared to those with much lower whole-grain intakes (average of 0.2 daily servings) (27).

There are a number of possible explanations for the cardioprotective effects associated with higher intakes of whole grains and lower intakes of refined grains. Compared to refined grains, whole grains are richer in nutrients associated with cardiovascular risk reduction, including folate, magnesium, and potassium. Although wheat fiber has not been found to lower serum cholesterol levels, numerous clinical studies have demonstrated that increasing oat fiber intake results in modest reductions in total and LDL-cholesterol (28). Barley intake can also lower serum total and LDL cholesterol (29, 30), and the FDA health claim was recently amended to include soluble fiber from barley (31). Whole grains are also sources of phytosterols, compounds that decrease serum cholesterol by interfering with the intestinal absorption of cholesterol (32). The relatively low glycemic index values of whole grains compared to refined grains may also play a role in decreasing the risk of heart disease. Substituting whole-grain products for refined-grain products in one’s diet decreases dietary glycemic load. Recent results from large prospective studies suggest that low-glycemic load diets are associated with lower coronary heart disease risk than high-glycemic load diets (33). For more information, see the article on Glycemic Index and Glycemic Load. Further, whole-grain intake was recently associated with a reduced risk of hypertension, a risk factor for cardiovascular disease (34, 35).

Cancer

Although the protective effects of whole grains against various types of cancer are not as well established as those against diabetes and cardiovascular disease, numerous case-control studies have found inverse associations between various measures of whole-grain intake and cancer risk (36-38). A meta-analysis of 40 case-control studies examining 20 different types of cancer found that people with higher whole-grain intakes had an overall risk of cancer that was 34% lower than those with lower whole-grain intakes (36). These studies generally used some measure of whole-grain bread intake to assess intake of whole grains, although a series of Italian case-control studies also assessed the intake of whole-grain pasta (37). Higher intakes of whole grains were most consistently associated with decreased risk of gastrointestinal tract cancers, including cancers of the mouth, throat, stomach, colon, and rectum. A prospective cohort study that followed more than 61,000 Swedish women for 15 years found that those who consumed more than 4.5 servings of whole grains daily had a risk of colon cancer that was 35% lower than those who consumed less than 1.5 servings of whole grains daily (39). A much larger prospective study in a cohort of 291,988 men and 197,623 women found that whole-grain intake was inversely associated with risk of colorectal cancer, especially rectal cancer (40). Specifically, compared to those in the lowest quintile of whole-grain intake, those in the highest quintile of whole-grain intake had a 21% and 36% lower risk of developing colorectal cancer or rectal cancer, respectively (40). Intake of whole-grain foods was related to a 41% lower risk for cancer of the small intestine in this cohort, but the trend did not reach statistical significance (41). However, not all cohort studies have reported that whole grains are protective against intestinal cancers (42, 43).

In contrast to refined-grain products, whole grains are rich in numerous compounds that may be protective against cancer, particularly cancers of the gastrointestinal tract (44). Higher fiber intakes are known to speed up the passage of stool through the colon, allowing less time for potentially carcinogenic compounds to stay in contact with cells that line the inner surface of the colon. Lignans in whole grains are phytoestrogens, which may affect the development of hormone-dependent cancers. Phenolic compounds in whole grains may modify signal transduction pathways that promote the development of cancer or bind potentially damaging free metal ions in the gastrointestinal tract.

Intestinal health

Diets rich in whole grains and fiber help prevent constipation by softening and adding bulk to stool and by speeding its passage through the colon(45). Such diets are also associated with decreased risk of diverticulosis, a condition characterized by the formation of small pouches (diverticula) in the colon. Although most people with diverticulosis experience no symptoms, about 15-20% may develop pain or inflammation, known as diverticulitis. Diverticulitis was virtually unheard of before the practice of milling (refining) flour began in industrialized countries, and the role of a low-fiber diet in the development of diverticular disease is well-established (46). Although high-fiber diets are recommended for people with constipation and diverticulosis, people with diverticulosis are sometimes advised to avoid eating small seeds and husks to prevent them from becoming lodged in diverticula and causing diverticulitis. However, it should be noted that no study has ever shown that avoiding seeds or popcorn reduces the risk of diverticulitis in an individual with diverticulosis (46).

Intake Recommendations

Whole-grain intakes approaching three servings daily are associated with significant reductions in chronic disease risk in populations with relatively low whole-grain intakes. One of the objectives of the US Department of Health and Human Services’ disease prevention agenda, Healthy People 2010, is to increase the proportion of people in the US who consume three servings of whole grains daily. However, most Americans consume less than one serving daily (2). Table 1 provides some examples of a serving of whole grains. The 2005 Dietary Guidelines for Americans recommend consuming three or more servings of whole-grain products daily. In view of the potential health benefits of increasing whole-grain intake, three daily servings of whole-grain foods should be seen as a minimum, and whole-grain foods should be substituted for refined carbohydrates whenever possible.

Examples of one serving of whole grains

1 slice of whole-grain bread

½ whole-grain English muffin, bagel, or bun

1 ounce of ready to eat whole-grain cereal

½ cup of oatmeal, brown rice, or whole-wheat pasta (cooked)

5-6 whole-grain crackers

3 cups of popped popcorn

Increasing whole-grain intake

Finding whole-grain foods

Whole grains include amaranth, barley, brown rice, buckwheat (kasha), flaxseed, millet, oats, popcorn, quinoa, rye, spelt, triticale, whole wheat (wheat berries), and wild rice (47). Unfortunately, it is not always clear from the label whether a product is made mostly from whole grains or refined grains. Some strategies to use when shopping for whole-grain foods include:

Looking for products that list whole grain(s) as the first ingredient(s).

Looking for whole-grain products that contain at least 2 grams of fiber per serving, since whole-grain foods are rich in fiber.

Looking for products that display this health claim, “Diets rich in whole grain foods and other plant foods and low in total fat, saturated fat and cholesterol may help reduce the risk of heart disease and certain cancers.” Products displaying this health claim must contain at least 51% whole grain by weight (48).

Looking for whole-wheat pasta that lists whole-wheat flour as the first ingredient. Most pasta is made from refined semolina or durum wheat flour.

Some strategies for increasing whole-grain intake

Eat whole-grain breakfast cereals, such as wheat flakes, shredded wheat, muesli, and oatmeal. Bran cereals are not actually whole-grain cereals, but their high fiber content also makes them a good breakfast choice.

Substitute whole-grain breads, rolls, tortillas, and crackers for those made from refined grains.

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